Power Engineering - 1 - Mechanical Engineering MCQ

In a thermal power station fuel such as coal, oil or gas is burned in a furnace to produce heat - chemical to heat energy.

• This heat is used to change water into steam in the boiler
• The steam drives the turbine - heat to kinetic energy (Mechanical energy)
  • This drives the generator to produce electricity - kinetic to electrical energy

• Fire tube boiler: Hot flue gases are inside the tubes and the water surrounds the tubes. Example: Cochran, Lancashire and Locomotive boilers
• Water tube boiler: The water is inside the tubes and hot gases surround them. Example: Babcock and Wilcox boiler, Stirling boiler
• Externally fired boilers: Fire is outside the shell. Example: Babcock and Wilcox boiler, Stirling boiler (water tube boilers)
• Internally fired boilers: Furnace is located inside the boiler shell. Example: Cochran, Lancashire boiler
• High pressure boilers: It produce steam at a pressure of 80 bar and above. Example: Babcock and Wilcox boiler, Velox, Lamont, Benson Boilers
• Low pressure boilers: It produce steam at a pressure below 80 bar are called low pressure boilers. Example: Cochran, Cornish, Lancashire and Locomotive boilers

The draught is one of the most essential systems of thermal power plant which supplies required quantity of air for combustion and removes the burnt products from the system.

Natural draught: A draught produced by a chimney due to the difference of densities between the hot gases inside the chimney and cold atmospheric air outside it.

Induced draught: The air pressure at the fuel bed is reduced below that of atmosphere by means of a fan placed at or near the bottom of the chimney.

Steam jet draught:

a) Induced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the chimney.

b) Forced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the ashpit under the fire grate of the furnace. Example: Locomotive boiler

Balanced draught: It is a combination of induced and forced draught.

• The curve which shows the variation of load on the electrical power station with respect to time is known as load variation curve or load curve.
• The Daily load curve gives the information of the load on the power station during different running hours of the day.
• The area under the daily load curve gives the total units of electrical energy generated.

           Units Generated/day=Area under daily load curve (kW)

• The maximum demand of the station on that day is found from the highest point of the daily load curve.
• Average Load=Area under the daily Load Curve (kWh)/24 hrs
• Load Factor= Average Load/Maximum Load

When a flowing fluid at a given pressure and temperature passes through a restriction (such as the throat of a convergent-divergent nozzle or a valve in a pipe) into a lower pressure environment the fluid velocity increases. Mass flow continues to increase with decreasing the exit pressure. Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment.

A nozzle is said to be chocked when flow rate through it is maximum and at throat of nozzle M = 1

In water tube boilers, water is contained in many small diameter tubes; therefore, the heating surface of a water tube boiler is more than that of the fire tube boiler. The relatively large heating surface of the water tube boiler increases the evaporation rate. The increased rate of evaporation of the water tube boiler makes it more suitable for large power plants.

Blade or diagram efficiency is defined as the ratio of work done on the blades to the energy supplied to the blades.

Blade speed ratio is the ratio of blade speed to a steam speed

η_b will be maximum when:

α₁ is of the order 18° to 22°

Convergent - Divergent nozzles are used to increase the flow of gas to supersonic speeds (as in the case of rockets). Their cross - sectional area first decreases and then increases. The area where the diameter is minimum is called the throat.

As the gas enters the converging section, its velocity increases, considering the mass flow rate to be constant. As the gas passes through the throat, it attains sonic velocity (mach number = 1). As the gas passes through the divergent nozzle, the velocity increases to supersonic (mach number >1)

The flow rate is maximum for a given nozzle if flow is sonic at the throat. This condition is achieved by managing the back pressure.

For nozzle: Steady flow energy equation is:

Boiler mountings are the components generally mounted on the surface of the boiler to have safety during operation. These are the essential parts of the boiler, without which the boiler operation is not possible. The following are the important mountings of the boiler : Water level indicator, Safety valve, Pressure gauge, Steam stop valve, Feed check valve, Main hole, Blow off cock.

Boiler Accessories are those devices which are installed with a boiler to increase the efficiency of the boiler. These are not the essential part of the boiler. The following are the important accessories of the boiler are: Economiser, Air pre-heater, Super heater, Steam drier or separator, Steam trap

Degree of reaction or reaction ratio (R) is defined as the ratio of the static pressure drop in the rotor (moving blade) to the static pressure drop in the stage (both fixed blade and moving blade) or as the ratio of static enthalpy drop in the rotor to the static enthalpy drop in the stage.

Degree of reaction=

Superheaters: It is used to dry the wet steam and raise the temperature of the steam above its saturation temperature. It is generally placed in the path of the furnace gases so as to utilize the heat from the hot flue gases.

Economizer: It is also known as feed water heater. It is a device in which the waste heat of the flue gases is utilised for heating the feed water.

Air Preheater: It is used to increase the temperature of air before it enters the furnace. It is generally placed after the economiser; so that the flue gases pass through  the economiser and then to the air preheater.

Electrostatic precipitator, also called electrostatic air cleaner, a device that uses an electric charge to remove certain impurities—either solid particles or liquid droplets—from air or other gases in smokestacks and other flues.

Boiler Drum is a pressure vessel it is used to Separate steam and water mixture.

So super heaters, economiser and air preheater are used for part recovery of heat from the flue gases.

n order to maintain the continuous flow of fresh air into the combustion chamber, it is necessary to exhaust the products of combustion from the combustion chamber of a boiler. A pressure difference has to be maintained to accelerate the products of combustion to their final velocity and to overcome the pressure losses in the flow system. This pressure difference so maintained is called draught.

The natural/chimney draught depends on

• the atmospheric temperature
• flue gases temperature leaving the furnace
• and height of the chimney

Rankine cycle is a reversible cycle which have two constant pressure and two isentropic processes. These are four processes in the Rankine cycle:

Process 1 – 2: Isentropic compression

Working fluid is pumped from low to high pressure.

Process 2 – 3: Isobaric heat addition

The high-pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour.

Process 3 – 4: Isentropic expansion

The dry saturated vapour expands through a turbine, generating power.

Process 4 – 1: Isobaric heat rejection

The wet vapour then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid.

The efficiency of the Rankine cycle is then given by:

Rankine cycle is a reversible cycle which have two constant pressure and two isentropic processes. These are four processes in the Rankine cycle:

Process 1 – 2: Isentropic compression

Process 2 – 3: Isobaric heat addition

Process 3 – 4: Isentropic expansion

Process 4 – 1: Isobaric heat rejection

Apply Steady Flow energy equation:

For boiler: Q_in = h₃ – h₂

For turbine: W_turb,out = h₃ – h₄ (change of enthalpy between inlet and outlet)

For Condenser: Q_out = h₄ – h₁

For Pump: W_pump,in = h₂ – h₁

Concept:

If there are N number of stages, then condition for minimum work required is:

Overall pressure ratio = (pressure ratio in each stage)^N

Calculation:

Pressure ratio in each stage

Delivery pressure of first stage is

= P₂ = 4P₁ = 4 × 1 = 4 bar

Diaphragm in steam turbines is a separating wall between rotors carrying nozzles. Additionally, as there is a significant pressure drop across a reaction turbine stage, the diaphragm also acts as a partition between the pressure stages.

Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment while upstream pressure is fixed.

For chocked flow in convergent – divergent nozzle, Mach number at throat is equal to 1 and the pressure at throat is equal to the critical pressure.

Critical pressure ratio for a choked nozzle:

Where p* is the critical pressure and p₀ is the inlet pressure